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January 12, 2007
about 3 years ago
I had trouble with the watch constantly resetting itself and the time while wearing it, so I eventually had to jumper across R1, the 100k resistor. If/when I interface it with a programmer I’ll have to remove it - temporarily!
No doubt this was due to the accumulation of static on the case itself causing the reset line to “flap”, and static electricity can be a real problem in the winter here in the low humidity of the western U.S. - particularly if you are wearing anything synthetic!
There still is a problem with the unit randomly lighting up and displaying the time, this also being due to static. Apparently some of the lines driving the display are held in a Hi-Z state and they can be dragged around, too, and causing the the same effect as pushing the button.
I suppose that the choices are to put a resistor on the line to drag it to the appropriate state when the display is dark (current would only be increased slightly while the display is lit) or, possibly reprogramming it to keep those pins in the “output” mode which may make it less static-sensitive. It may be that I also need to add a resistor to the button line (pin 4) to bolster the internal pull-up, but bench-testing indicated that these other lines were definitely static-sensitive!
When assembling the case I coated each acrylic piece with “Nu-Finish” car polish. Long having used this to treat other plastic pieces (CDs and their jewel cases, other plastic parts) it goes a long way in preventing scratches as its polymer bonds to the surface pretty well and decreases its coefficient of friction considerably. Doing this may contribute to the static problem, but acrylic is pretty good about that on its own, anyway!
Finally, as far as accuracy goes: Mine seems to be fast by about 30-45 seconds per month. I suppose that I could tweak the 32kHz oscillator a bit, but I’ve not really bothered. Being open-source, it would be pretty easy to add a “calibrate” mode to occasionally add/subtract a second to help compensate for the portion of drift that isn’t temperature-related.
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